The present invention is directed to testing of ISDN (Integrated Services Digital Network) switches (for example, at an ISDN central office), and, more particularly, to a traffic generator adapter to allow testing such ISDN switches using commercially available analog traffic generators.
ISDN is presently being promoted by telephone companies in an effort to improve quality, capacity, and variety of services. Preliminary ISDN specifications have been introduced by CCITT, the international communications standards committee. In the U.S. and Canada, subsets and minor variations of these standards are being defined by the major central office equipment vendors, such as AT & T and Northern Telecom.
The technical concept of ISDN is being accomplished by using the existing copper telephone lines to carry a digital signal instead of the usual analog voice (limited to 3000 Hz). The digital signal is transmitted at a rate of 160,000 bits per second (bps), offering two 64,000 bps channels (known as "B" channels) for voice or data calls, and one 16,000 bps channel (identified as the "D" channel) for establishing and maintaining the call connection to the network (and also used for sending data packets using the X.25 protocol) as a basic service. Implementation of ISDN requires new central office switching and transmission equipment, and sometimes dramatically different terminal equipment (e.g., telephones, modems, data terminals) at the user or subscriber end of the telephone line. At present, ISDN is starting limited technical field trials in several cities around the U.S., and is already being widely implemented in Europe.
A method was needed to test the new ISDN central office switches under simulated load conditions to make sure that they were working and could handle the number of calls expected when placed in operation. With the older style telephone switches, traffic generators were used to place a large number of calls through the switch. They would also monitor the time needed to complete the call, the quality of the connection and speed that the switch could handle simultaneous requests for service. Since the existing traffic generators were analog and the ISDN switches required a digital input, they could not be used to directly test ISDN switches. Another method was needed.
Accordingly, most of the ISDN switches now being installed includes self test capability. This function can test a large percentage of the switch hardware for proper operation including controller operation and the integrity of internal data paths. However, built-in self test equipment is not capable of providing actual inputs from external equipment. On the contrary, it only can simulate internal functions. It cannot test the response of the switch to actual off-hook and dialing signals from the customer's telephone, nor can it test the complete data path from end to end (telephone to telephone). In addition, there is no way for the customer to check that the internal self test provided by the switch manufacturer really provides a complete test of the switch. Accordingly, the present invention is directed to an adapter permitting the use of commercially available analog traffic generators to perform extensive testing of ISDN switches.
In addition, it is often necessary to identify the source of trouble in the event of problems in making or answering a voice or data call. As noted above, in basic ISDN service, the voice or data information is carried by one of two B channels on the ISDN circuit. The cell transitions to make a connection, or link, are established via the D channel. Messages sent to and from the central office on the D channel contain the information defining the status of the link. It is very important to known what part of the link activity is failing to determine if the trouble is in the local equipment or elsewhere. All D channel information may be displayed in the following formats:
1. Binary.
2. Hex.
3. Bus activation state.
4. OSI Layer 1 HDLC frames.
5. OSI layer 2 LAPD/LAPD decoder frames.
6. OSI layer 3 Q.931 and decoder X.25 frames.
The present invention also includes a D channel monitor feature to help isolate the location of any problems quickly and accurately.
FIG. 1 shows a basic ISDN terminal which the present invention can be used in conjunction with. The terminal is divided into functional groupings. By separating functions conceptually in this way, it is possible to identify and specify the interfaces between them. A description of the functional groups is as follows, noting that this description is confined to the functions of basic access.
NT1-Network Termination 1. This is the group of functions which terminate the transmission line. As such, it is seen in the CCITT recommendations as belonging to the network provided, i.e., the owner of the transmission line. The functions of the NT1 are described as:
Line transmission termination. PA1 Line maintenance and performance monitoring. PA1 Timing. PA1 Power transfer, extracting power from the line to drive at least the "wake-up" portion of the terminal. PA1 Parts of the multiplexing functions. PA1 Termination of the T interface which may include multi-drop termination and associated contention resolution functions. PA1 Protocol handling or handling that part of the protocol associated with information transfer across a network. PA1 The higher-level parts of the multiplexing function. PA1 Switching and concentration functions. PA1 Maintenance functions. PA1 Interface functions to the S and T interfaces.
NT2-Network Termination 2. This is the group of functions which give the terminal its particular "character". An NT2 could be a PABX if access is primary, a LAN or a terminal controller. The functions of the NT2 are described as:
The NT2 functional group may be more or less complex depending on the application. The range extends from the quite complex function of a PABX down to relatively simple functions required for a time division multiplexer. In specific, simple cases all the functions may be adequately performed by NT1, and NT2 becomes merely physical connections.
TE-Terminal Equipment. This is the device itself. It could be a digital telephone, a CADCAM workstation, a computer terminal, etc.
TE1-Terminal Equipment Type 1. Such terminal equipment complies with ISDN user-network interface recommendations and therefore supports interface S.
TE2-Terminal Equipment Type 2. Such a terminal supports the same functions but does not comply with the ISDN user-network interface recommendations. It must therefore interface with the ISDN access via a
TA Terminal Adapter which converts the non-ISDN interface functions into ISDN acceptable form at reference points S or T.
This description has largely revealed the nature of the interfaces. A further description is as follows:
Reference point T (T for Terminal) Separates the network provider's equipment from the user equipment. Provides a standardized interface between equipment, sending and receiving, validating and timing information to the network and to terminal equipment devoted to the use of this information.
Reference point S (S for System) Separates the user terminal equipment from the network functions of the terminal.
Reference point R (R for Rate) Provides a non-ISDN interface between non-ISDN compatible user equipment and adapter equipment. Such an interface may well comply with one of the CCITT X series interface recommendations.
Reference point U (U for User) Interface between NT1 and the transmission line.
The terminology from FIG. 1 will be used throughout the rest of the specification. This terminology is presently well accepted standard terminology, as discussed, for example, in the textbook, "The Integrated Services Digital Network: from Concept to Application", by John Ronayne, published by John Wile & Sons, Inc., 1988, which is hereby incorporated by reference.